The lost generation of Calanus pacificus: Is the diatom effect responsible?

Limnology and Oceanography (Impact Factor: 3.62). 01/2007; DOI: 10.2307/4502359

ABSTRACT An apparent mismatch between periods of high reproduction (spring) and high copepodite abundance (autumn) has been observed for the copepod Calanus pacificus in Dabob Bay, Washington, U.S.A. This persistent pattern leads to a lost generation of progeny that are produced in spring but do not recruit to the juvenile and adult populations, and this is likely due to a combination of factors including advective losses, predation mortality, and nonviability of progeny. Here we test the hypothesis that observed detrimental effects of diatoms on the viability of copepod embryonic and naupliar stages (the diatom effect) are the primary reason for the observed patterns of reproduction and abundance. Furthermore, we test how assumptions about egg production rate and naupliar viability can affect calculations of copepod recruitment. To test these hypotheses, we developed a numerical model to quantitatively explore how certain parameters may have affected the population of C. pacificus population in Dabob Bay. Predation mortality was the most significant contributor to population losses, while advective losses and naupliar viability were of lesser importance, and the model results were more sensitive to parameterization of naupliar viability than egg production rate. Although brief instances of low naupliar viability caused a 25-30% reduction in cumulative stage I copepodite abundance over time when compared with the assumption of persistent high naupliar viability, the lost generatio no fC. pacificus in Dabob Bay is likely due to predation mortality, not the diatom effect. In marine planktonic copepods, egg production rate, viability of the offspring, predation rate, and advective flux rate are factors affecting recruitment of copepodite stages from spawned eggs. Since 1994, there have been a number of laboratory studies examining the viability of copepod progeny when females are fed different food types, and many of these studies suggest that diets high in diatoms can reduce reproductive success compared to diets with alternative or mixed prey types (see reviews in Paffenhofer 2002; Ianora et al. 2003). This phenomenon has been termed the ''diatom effect'' (cf. Halsband-Lenk 2005), but its relevance to field populations has been ambiguous. Egg production rate has been widely shown to be positively correlated with concentration of chlorophyll (e.g., Bunker and Hirst 2004); however, there does not appear to be any relationship between egg hatching success (the proportion of spawned eggs that hatch, which is often .80% )a nd



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